1. Field of the Invention
Apparatus and process for treating wood pulp with oxygen.
2. Review of the Prior Art
The following definitions will be used in this application.
Pulping is the changing of wood chips or other wood particulate matter to fibrous form. Chemical pulping requires cooking of the chips in solution with a chemical, and includes partial removal of the coloring matter such as lignin associated with the wood.
Bleaching is the treatment of cellulosic fibers to remove or alter the coloring matter associated with the fibers to allow the fiber to reflect white light more truly.
The standard symbols for pulping and bleaching sequences are:
S=Sulfite
K=Kraft
So=Soda
C=Chlorine
H=Sodium or calcium hypochlorite
E=Alkali extraction, usually with sodium hydroxide
D=Chlorine dioxide
P=Alkaline peroxide
O=Oxygen
A=Acid pretreatment or post treatment
Consistency is the amount of pulp fiber in a slurry, expressed as a percentage of the total weight of the oven dry fiber and the solvent, usually water. It is sometimes called pulp concentration.
The consistency of the pulp will depend upon the type of dewatering equipment used. The following definitions are based on those found in Rydholm Pulping Processes, Interscience Publishers, 1965, pages 862-863 and TAPPI Monograph No. 27, The Bleaching of Pulp, Rapson editor, The Technical Association of Pulp and Paper Industry, 1963, pages 186-187.
Low consistency is from 0-6%, usually between 3 and 5%. It is a suspension that is pumpable in an ordinary centrifugal pump and is obtainable using deckers and filters without press rolls.
Medium consistency is between 6 to 20%. Fifteen percent is a dividing point within the medium-consistency range. Below 15% the consistency can be obtained by filters. This is the consistency of the pulp mat leaving the vacuum drum filters in the brownstock washing system and the bleaching system. The consistency of a slurry from a washer, either a brownstock washer or a bleaching stage washer, is 9-13%. Above 15%, press rolls are needed for dewatering. Rydholm states that the usual range for medium consistency is 10-18%, while Rapson states it is 9-15%. The slurry is pumpable by special machinery even though it is still a coherent liquid phase at higher temperatures and under some compression.
High consistency is from 20-40%. Rydholm states that the usual range is 25-35% and Rapson states that the range is from 20-35%. These consistencies are obtainable only by presses. The liquid phase is completely absorbed by the fibers, and the pulp can be pumped only very short distances. For practical purposes, it is nonpumpable.
Pulp quantity is expressed in several ways.
Oven dry pulp is considered to be moisture free or bone dry. Its value is determined by drying the pulp in an oven at a temperature of 100.degree. to 105.degree. C. until it reaches constant weight. It usually is considered to have reached constant weight after 24 hours in the oven.
Air dry pulp is assumed to have a ten percent moisture content. One air-dry ton of pulp is equal to 0.9 oven-dry tons of pulp.
It also may be helpful in the understanding of the prior art to describe a typical pulp mill and relate the prior art oxygen bleaching systems as well as the present invention to this typical mill.
FIG. 1A-1C is a diagram of a typical pulp mill. In the mill the means of transporting chips or pulp from one operation to another will depend upon the consistency of the pulp and the location of the equipment. The transportation means may be a conveyor or a chute if the consistency is too high for the pulp or chips to be pumped, or a pipe if the pulp is capable of being pumped.
Chips 10, process water 11, steam 12 and pulping chemicals 13 are placed in a digester 14. The wood chips 10 may be treated prior to entering the digester 14. This is optional. Exemplary of such treatment are presteaming of the chips in a steaming vessel or impregnation of the chips with the digestion chemicals in an impregnation vessel prior to entering the digester. The chemicals 13 will depend on the process being used, be it sulfate, sulfite, or soda, and the digester 14 may either be batch or continuous in operation. A continuous digester is shown. The chips will be cooked under appropriate conditions within the digester. These conditions, which depend on the species of chip and the type of pulping used, are well known.
The products of the digestion process are the delignified or partially delignified wood chips, the spent pulping chemicals, and the lignin and carbohydrate products which have been removed from the wood chips in the digestion process. The treatment of the chips, after cooking, will depend in part on the type of digester being used. A major portion of the spent pulping chemicals and lignin products are removed from the chips prior to further processing. In the continuous digester shown, the chips are washed in the washing section of the digester. This is indicated by process water 15 entering and effluent stream 16 leaving the washing stage of digester 14. The effluent 16 will consist of the lignin and carbohydrates which have been removed from the chips during the digestion process and the spent digestion chemicals. This effluent will be carried to a treating facility. In the case of kraft or sulfate pulp this would be a recovery system in which the liquor is burned to recover the digestion chemicals for reuse.
This washing would not take place in a batch digester. In a batch system, all the washing would occur in the following brownstock washing system.
Following this treatment, the chips will pass from the digester 14 through the blow line to storage or blow tank 22. It is customary in pulp mills to have storage tanks between separate processes so that the entire mill will not shut down if one section of the mill is shut down. Storage tank 22 is one such tank. It would be between the digester stage and the subsequent washing or bleaching stages. The storage tank 22 is open to the atmosphere and so is at atmospheric pressure.
Tank 22 may be a diffusion washer instead of a storage tank. Diffusion washers are described in Rydholm Pulping Processes, Interscience Publishers, 1965, pages 725-730 and illustrated in FIG. 10.14 on page 728. The diffusion washer would be followed by a storage tank.
The material passing through the blow line is a slurry which contains the remaining lignin and carbohydrates, the spent digestion chemicals, and the fibers formed from the chips as they are blown from the digester. The chips will be formed into fibers when the pressure on the chips is partially released, usually at the outlet of digester 14. The slurry will still be under some pressure to move it through the blow line. If the digester is continuous, then additional fiberizing may be done by a refiner, or refiners, in the blow line. The refiners will fiberize the large particles that have not been reduced to fibers earlier in the process. In the present diagram, two refiners--18 and 19--are shown. In the two-refiner system, the first refiner 18 does course refining and the second refiner 19 does fine refining. The refiners are optional. They are usually encountered in a linerboard mill. The digester in a bleached pulp mill normally would not have refiners in the blow line. Neither would they be used with a batch digester.
The blow line is shown in three sections--section 17 between the digester 14 and refiner 18; section 20 between the refiners 18 and 19; and section 21 between the refiner 19 and the storage tank 22.
From the storage tank 22 the fibers and liquor are carried by pump 23 through line 24 to the washers and screens. The system will be described by following the pulp through the system, and then following the wash water through the system.
The pulp slurry is first carried to the brownstock washers 28 where the rest of the lignin and chemicals are removed from the fibers. Four washers are shown. This is the number that would normally be used with a batch digester. The washing section in a continuous digester would replace the first two brownstock washers. Each of these washers is usually a vacuum or pressure drum washer or vacuum or pressure drum filter and the operation of each is the same. The operation of a vacuum or pressure drum washer will be described. Some of the washers may, however, be diffusion washers. In a diffusion washer, the pulp slurry would not be diluted prior to entering the washer.
The pulp slurry from line 24 enters the vat 30 of washer 31. The vacuum drum 32 revolves through the vat, and the vacuum pulls the fibers in the slurry onto the outer surface of the filter drum and holds the fibers, in mat form, against the surface while pulling the liquor or filtrate through the filter cloth to the interior piping of the vacuum drum to be discharged as effluent. The revolving drum carries the fiber mat from the vat past a bank of washer heads that spray a weak filtrate onto the mat to displace the liquor from the mat. The vacuum also pulls this displaced liquid into the interior piping of the drum. The consistency of the mat leaving a washer, either the brownstock washers described here or the bleach washers described later, will usually be between 8 to 15%.
The pulp mat 33 is removed from the face of the drum 32 by a doctor blade, carrier wires or strings between the drum and the mat, rolls or any other standard manner and carried to the vat 50 of the second brownstock washer 51. Again, the fibers are picked up on vacuum drum 52. The pulp mat 53 is washed with still weaker filtrate, removed from the vacuum drum 52 and carried to the vat 70 of brownstock washer 71. The operation of this washer is the same as the others, the vacuum drum being 72 and the mat 73. The mat 73 is carried to the vat 90 of the last brownstock washer 91. Again, the operation of this washer is the same as the others, the vacuum drum being 92 and the mat 93.
From the brownstock washers the pulp mat 93 is carried to storage tank 110 with the aid of thick stock pump 96. In the lower section of tank 110, the pulp is diluted and then carried through line 111 by pump 112 to screens 113 in which the larger fiber bundles and knots are removed. The bundles and knots 114 are carried to further treatment by suitable transportation means.
The pulp 115 is carried from the screens 113 to the vat 120 of decker 121 in which additional water is removed. The operation of the decker is similar to that of the washers. Washing showers may or may not be used in the decker. The vacuum drum is 122 and the pulp mat is 123. The pulp 123 is carried by thick stock pump 126 to a high-density storage tank 140 in which it is stored until it is bleached.
The liquor or filtrate from the vat 120 and the mat 123 flows through piping which extends radially from the vacuum chambers at the surface of the vacuum drum 122 to a pipe in the central shaft of the rotating drum. This liquor or filtrate passes through the central pipe and an external line 128 to a filtrate storage tank or seal tank 129. The tank 129 is called both a storage tank and a seal tank because it acts both to store the filtrate for further use and to seal the vacuum drum 122 from the outside atmosphere to maintain the lower pressure of the vacuum system within the drum.
The filtrate from tank 129 may be handled in several ways. Several of the uses may occur simultaneously. Although the following description is specific to the effluent from tank 129, it is also illustrative of how the effluent from any of the washers in brownstock washing system 28 would be handled.
First, the filtrate from tank 129 is reused to reduce the consistency of the pulp slurry either entering the decker 121, entering the screens 113 or leaving storage tank 110. Line 130 carries the filtrate to lines 131, 133 and 135. Line 131 and pump 132 carry the filtrate back to screened pulp 115 to reduce the consistency of the pulp slurry entering vat 120 to around 11/2%. Line 133 and pump 134 carry the filtrate back to line 111 to reduce the consistency of the pulp slurry entering the screens 113 to from 0.2 to 2%. Line 135 and pump 136 carry the filtrate back to storage tank 110 to reduce the consistency of the pulp slurry leaving the tank to around 5%.
Second, the filtrate not reused for dilution may be taken to an effluent treatment system by line 130 and effluent line 29. This treatment may include combining the effluent with the effluent in line 16, or carrying the effluent directly to the cooking liquor recovery system. It should be understood that in a batch digester system the digester effluent is recovered completely from the brownstock washing system while in a continuous digestion system only a portion of the digester effluent would be recovered from the brownstock washers.
All of the remaining filtrate would be handled as effluent if counterflow washing, to be described next, is not used. Some of the filtrate may be handled as effluent even if counterflow washing is used.
Third, the filtrate from tank 129 may be used as wash water in the brownstock washing system 28 in a counterflow washing system. In this system, the filtrate flow is counter to the flow of pulp. The line 137 and pump 138 carry the filtrate back to brownstock washer 91 for use as wash water. The filtrate is sprayed on the pulp mat by washer heads 95 and displaces the liquor within the mat. This filtrate may also be sprayed on the carrier wires, strings or rolls after the pulp mat is separated from them to remove any pulp fibers that cling to the wires, strings or rolls if water instead of air is used for this operation. This is done by cleanup washer 94. Additional water may be required to supplement the filtrate. This is provided through process water line 97.
The flow of filtrate through brownstock washer 91 is the same as the flow through decker 121. The liquor, either from the mat or the vat, is carried through internal piping to line 98 and through line 98 to filtrate storage tank or seal tank 99. Again, the filtrate from the seal tank 99 may be handled in a number of ways. Line 100 would carry it to effluent line 29. Line 101 and pump 102 would carry the filtrate to pulp 73 to reduce the consistency of the pulp slurry to 11/2% to 31/2% as it enters vat 90. Line 103 and pump 104 would carry the filtrate to brownstock washer 71 to be used as wash water.
The process in brownstock washers 71, 51 and 31 are, for the most part, identical to the process in brownstock washer 91 so the parts are similarly numbered. The washer heads are 75, 55 and 35 respectively. The cleanup washers are 74, 54 and 34 respectively. The filtrate lines are 78, 58 and 38 and the filtrate storage or seal tanks are 79, 59 and 39. The filtrate lines from the seal tanks to effluent line 29 are 80, 60 and 40.
The consistency of the slurry entering any of the vats 70, 50 or 30 should be 11/2% to 31/2%. The lines and pumps carrying the filtrate to the pulp to reduce the consistency of the slurry entering a vat are 81 and 82, 61 and 62, and 41 and 42, respectively. The counterflow wash water lines and pumps are 83 and 84, and 63 and 64.
In brownstock washer 31, line 43 and pump 44 carry the filtrate into storage tank 22 to reduce the consistency of the pulp slurry in the bottom of the tank to 2 to 31/2% before it exits the tank.
In each of the brownstock washers, there is a possibility that additional process water may be needed to supplement the filtrate being used as wash water. Lines 77, 57 and 37 are for this purpose. These lines would provide all the wash water to the individual washers if the counterflow system described above is not used and parallel flow washing is used instead.
The washed pulp which has passed through the brownstock washing system 28, the screens 113 and decker 121 remains in storage tank 140 until it is carried into the bleaching system.
The bleaching process in FIG. 1 will also be described by following the pulp stream through the bleaching system from washed pulp to bleached pulp and then by following the wash water from its entry into the process through to bleach plant effluent. The particular bleaching sequence illustrated is D.sub.c EDED. The process conditions are taken from the TAPPI monograph The Bleaching of Pulp noted earlier.
There are many other bleaching sequences which could be used. Listings of these sequences may be found in the standard texts. As a rule, the first stage is chlorine and subsequent stages use chlorine dioxide, hydrogen peroxide, or a hypochlorite. These stages are interspersed with alkali extraction stages.
The pulp stored in high-density tank 140 normally is at a consistency of approximately 9 to 15%. This pulp slurry is carried from tank 140 through line 141 to tank 146 by pump 142. The pulp in line 141 is diluted with additional water or filtrate to a consistency of around 5%. In mixer 144 in line 141, the slurry is mixed with chlorine dioxide from line 145 as the D step of the first stage D.sub.c bleach. If the first stage is to be chlorine alone, then this step would be omitted. The treated dilute slurry enters storage tank 146 in which the chlorine dioxide reacts with the unbleached pulp. The size of this tank will depend upon the amount of pulp being treated and the time of the chlorine dioxide treatment. The time of this initial treatment normally is one to five minutes. The slurry exits the tank into line 150 and is treated with chlorine.
Chlorine from line 151 and process water from line 152 are mixed in aspirator 153 and the diluted chlorine flows through line 154 to mixer 155 in which the chlorine is mixed with the dilute pulp slurry in line 150. The treated slurry is moved by pump 156 through line 150B into chlorine bleaching tower 157. The tower 157 is sized to allow the chlorine to react with the extraneous matter in the unbleached pulp. This retention or reaction time will depend, in part, on the water temperature. At minimum temperature, the TAPPI monograph suggests a retention time of about 45 to 60 minutes for sulfite pulp, and 60 to 90 minutes for kraft pulps. The treated slurry exits tank 157 and is carried through line 158 by pump 159.
The slurry in line 158 is combined with additional water or filtrate to reduce the consistency to about 1 to 11/2%. This dilute slurry flows into vat 160 of washer 161. Again a vacuum drum washer or filter is shown. The operation of this washer is the same as that of the brownstock washers. The vacuum drum 162 revolves through the vat. The vacuum pulls the fibers in the slurry onto the outer filter surface of the drum and holds them against the surface, forming a mat, while pulling the liquid or filtrate through the filter cloth to the interior piping of the vacuum system in the drum to be discharged as effluent. The revolving drum 162 carries the fiber mat from the vat past a bank of washer heads which spray water or weak filtrate onto the mat to displace reaction products and unreacted chlorine entrained in the mat.
The pulp mat 163 is removed from the face of drum 162. The means of removal is the same as in the brownstock washers--a doctor blade, carrier wires or strings between the drum and the mat, rolls or in any other standard manner. The pulp mat 163 is moved to mixer 166. This movement usually is by gravity fall through a chute from the washer to the mixer.
Prior to leaving washer 161, the pulp mat 163 is impregnated with the caustic or alkali extraction solution from line 167. A sodium hydroxide solution is usually used. The alkali solution is applied to the mat just as it is leaving the vacuum drum 162 so that the solution will penetrate the pulp mat but not be carried through the mat into the washer effluent. The amount of alkali added, expressed as sodium hydroxide, will be 0.5 to 7% of the oven-dry weight of the pulp. The alkali may be added to the pulp in the steam mixer 166 instead of at the washer 161.
In steam mixer 166 the treated mat is mixed with steam from line 168 to raise the temperature of the pulp to approximately 62.degree. C. The heated slurry is carried through line 169 into extraction tower 173 by high-density pump 170. In some cases transfer to the extraction tower is by gravity. The extraction tower may be downflow or upflow. The high-density pump 170 for either an upflow or a downflow tower may be at the base of the tower. The pulp would then be carried to the top of a downflow tower by an external line. The location of the pump in the plant is a matter of convenience. Support of the pump and access to the pump for maintenance are primary considerations. The slurry remains in tower 173 to allow the extraction solution to react with and extract the chlorinated materials from the pulp. This time may be one to two hours.
Before leaving the extraction tower, the pulp slurry is mixed with water or filtrate in dilution zone 174 to reduce its consistency to approximately 5%. The slurry is carried by line 175 and pump 176 from dilution zone 174 to the vat 180 of washer 181. Washer 181 is shown and described as a vacuum or pressure drum washer but it may be a diffusion washer. During its passage through line 175, the slurry is further diluted with water or filtrate until its consistency is approximately 1 to 11/2% when it reaches the vat 180. The operation of washer 181 is identical to that of washer 161. The fibers are picked up on the revolving drum 182, washed and removed as pulp mat 183.
The pulp is then moved to steam mixer 186 of the chlorine dioxide stage. This transfer may again be be gravity drop through a chute. Prior to leaving washer 181, the mat 183 is treated with a slight amount of alkali from line 187. A sodium hydroxide solution is usually used. It is added to the mat at a point on the drum which will allow the solution to stay in the mat and not pass into the filtrate. The purpose of this treatment is not further extraction but adjustment of the pH of the pulp prior to being treated with chlorine dioxide. The pH of the pulp should be in the range of 5 to 7, preferably 6, for optimum brightness when bleaching with chlorine dioxide. The alkali may be added in the steam mixer 186 instead of the washer 181.
In steam mixer 186 the pulp 183 is mixed with steam from line 188. The pulp will have a consistency of approximately 1% less than from the washer when it leaves a steam mixer.
The pulp leaves steam mixer 186 and is carried through line 189 by pump 190 to mixer 191 which it is combined with chlorine dioxide from line 192. It then enters chlorine dioxide tower 193. This tower usually is an upflowdownflow tower. The pulp remains in the tower long enough to allow the chlorine dioxide to react with it. The reaction is about complete after one hour but normally continues for up to four hours. Consequently, the retention time in the tower is usually four hours. Prior to leaving the tower, the slurry is diluted to a consistency of about 5% in dilution zone 194. It is also treated with a small amount of sulfur dioxide or alkali from line 197. The sulfur dioxide or alkali reacts with any excess chlorine dioxide so there will be no free chlorine dioxide emanating from the washer or the pulp leaving the washer.
This diluted slurry is then carried by line 195 and pump 196 to vat 200 of washer 201. During its passage through line 195, the slurry is again diluted to a consistency of about 1 to 11/2% when it reaches vat 200, and again treated with additional sulfur dioxide from line 198. The pulp is picked up on vacuum drum 202, and the reaction products and unreacted bleaching chemicals washed from it prior to being removed as pulp mat 203.
This pulp is moved to the steam mixer 206 of the second extraction stage, usually by gravity drop through a chute. Again, sodium hydroxide from line 207 is added on washer 201 or at the mixer 206, and in mixer 206 the treated pulp mat 203 is mixed with steam from line 208. This slurry is then carried through line 209 by pump 210 to extraction tower 213. The conditions in this extraction stage are the same as those in the first extraction stage. The tower may be downflow or upflow.
After the appropriate dwell time, the pulp enters dilution zone 214, and its consistency is reduced to approximately 5%. The pulp is then carried through line 215 by pump 216 to the vat 220 of washer 221. Washer 221 is also shown and described as a vacuum or pressure drum washer but it may be a diffusion washer. Again, it is diluted to a consistency of about 1 to 11/2% before entering the vat. The slurry is picked up by vacuum drum 222 and washed and discharged as pulp mat 223. If necessary, the pH of the pulp may be adjusted by treating the mat with sodium hydroxide from line 227. This may occur on the drum 222 or in the steam mixer 226.
The pulp enters the last chlorine dioxide stage. The conditions and flow in this stage are the same as in the first chlorine dioxide stage. The pulp is dropped into or carried to steam mixer 226, and mixed with steam from line 228. The slurry is carried through line 229 by pump 230 to mixer 231, mixed with chlorine dioxide from line 232 and carried into the chlorine dioxide tower 233, shown as an upflow-downflow tower, where it remains for one to four hours. The pulp then enters dilution zone 234 where its consistency is reduced to about 5%. It is also treated with a small amount of sulfur dioxide from line 237 to remove any excess chlorine dioxide.
The slurry is carried from dilution zone 234 through line 235 by pump 236. During its travel through line 235, the pulp is again treated with additional sulfur dioxide or alkali from line 238 to remove any free chlorine dioxide and is further diluted so that the slurry is at a consistency of about 1 to 11/2% when it reaches vat 240 of washer 241. It is picked up by vacuum drum 242, washed and discharged from the bleaching system as bleached pulp 243.
Pulp from the mat usually adheres to the wire or strings carrying the pulp mat from the washer and it is necessary to wash these fibers from the wires or strings into the vat prior to their contacting new fibers. This may be done by cleanup washer 164 on washer 161, cleanup washer 184 on washer 181, cleanup washer 204 on washer 201, cleanup washer 224 on washer 221, and cleanup washer 244 on washer 241. Air may also be used.
The passage of liquid through the washer is the same as in the brownstock washers. Wash water is sprayed onto the mat by the washer heads. This water displaces the entrained liquid within the pulp mat on the drum. The displaced liquid is carried through piping internally of the rotating vacuum drum to a pipe in the central shaft of the drum. Here, it is combined with the liquor being pulled into the drum from the washer vat. This combined liquor passes outwardly through a central pipe in the drum and an external line to a seal or storage tank which maintains the vacuum in the drum by providing a seal between the vacuum inside the drum and the ambient pressure externally of the drum.
In washer 161, process water from washer heads 251 passes into the central shaft of vacuum drum 162, and outwardly through external line 252 to seal or storage tank 253. In washer 181, the washer heads are 271, the external line is 272 and the seal or storage tank is 273. In washer 201, the washer heads are 291, the external line is 292, and the seal or storage tank is 293. In washer 221, the washer heads are 311, the external line is 312, and the seal or storage tank is 313, and in washer 241 the washer heads are 331, the external line is 332, and the seal or storage tank is 333.
The routes taken by the filtrate after it leaves the seal or storage tank are also the same as those in the brownstock washers.
First, the filtrate is used to dilute the slurry within the washing stage or a tower.
For example, the filtrate would dilute the pulp slurry being carried to the vat. The filtrate from the seal tank 253 of the chlorine stage washer 161 would be carried by line 255 and pump 256 into line 158 to be used to dilute the pulp slurry going to vat 160. In the same way, line 275 and pump 276, and line 277 and pump 278, carry the filtrate from the seal tank 273 of the first extraction stage washer 181 into line 175 to dilute the slurry going to vat 180; line 295 and pump 296, and line 297 and pump 298, carry the filtrate from the seal tank 293 of the first chlorine dioxide stage washer 201 into line 195 to dilute the slurry going to vat 200; line 315 and pump 316, and line 317 and pump 318, carry the filtrate from the seal tank 313 of the second extraction stage washer 221 into line 215 to dilute the slurry going to vat 220; and line 335 and pump 336, and line 337 and pump 338, carry the filtrate from the seal tank 333 of the second chlorine dioxide washer 241 into line 235 to dilute the slurry going to vat 240.
In the chlorine stage, line 259 and pump 260 also carry the filtrate to line 141 to dilute the pulp from highdensity storage.
In the extraction and chlorine dioxide stages, the filtrate is also supplied to dilute the slurry in the dilution zone of the tower in the stage. In the first extraction stage, the filtrate from seal tank 273 is carried into the dilution zone 174 by line 281 and pump 282. In the first chlorine dioxide stage, line 301 and pump 302 carry the filtrate from the seal tank 293 into the dilution zone 194. In the second extraction stage, line 321 and pump 322 carry the filtrate from the seal tank 313 into dilution zone 214, and in the second chlorine dioxide stage line 341 and pump 342 carry the effluent from the seal tank 333 into dilution zone 234.
Second, the filtrate not reused for dilution is discharged as effluent or to further processing as by line 254 from tank 253, line 274 from tank 273, line 294 from tank 293, line 314 from tank 313, and line 334 from tank 333. The effluent from the chlorine stage washer 161 is separate from the effluent from the other washers because of its high chlorine or salt content and its larger content of residual material. The other lines--274, 294, 314 and 324--discharge into effluent line 350.
All of the remaining filtrate would be handled as effluent if counterflow washing is not used. Some of the filtrate may be handled as effluent even if counterflow washing is used.
In the counterflow washing system shown, the wash water for the second chlorine dioxide washer 241 is process water from line 330; the wash water for the second extraction washer 221 is partly or wholly filtrate from the second chlorine dioxide washer 241 which is supplied from seal tank 333 by line 343 and pump 344; the wash water for the first chlorine dioxide washer 201 is partly or wholly filtrate from the second extraction washer 221 which is supplied from seal tank 313 by line 323 and pump 324; the wash water for the first extraction washer 181 is partly or wholly filtrate from the first chlorine dioxide washer 201 which is supplied from seal tank 293 by line 303 and pump 304; and the wash water for the chlorine washer 161 is partly or wholly filtrate from the first extraction washer 181 which is supplied from seal tank 273 by line 283 and pump 284. Any additional wash water would be supplied through lines 250, 270, 290 and 310. These lines would provide all the wash water to the individual washers if the counterflow system is not used and parallel flow washing is used instead.
The chemical, water and steam supplies to the system are shown in the upper section of FIG. 1. Process water is carried through line 360 to the various lines supplying water to the process, line 351 to the digester lines 11 and 15, lines 37, 57, 77 and 97 to the brownstock washers 28, line 152 to the chlorine aspirator 153, and lines 250, 270, 290, 310 and 330 to the bleach system washers. Chlorine is supplied through line 361 to line 151. Alkali line 362 supplies dilute alkali to lines 167, 187, 207 and 227. It is normally a 5-10% solution before entering line 362. Chlorine dioxide line 363 supplies a chlorine dioxide solution to lines 145, 192 and 232. Steam is supplied through line 364 to steam lines 12, 168, 188, 208 and 228. Sulfur dioxide is supplied to lines 197, 198, 237 and 238 from line 365.
Methods of measuring the practically and efficiency of a pulping or a bleaching process are the pulp yield, the physical properties of the pulp, the degree of pulp delignification, the pulp brightness, and the cost of obtaining the pulp.
Yield may be measured in two ways. The first is the amount, by weight, of carbohydrates and lignin returned per unit of wood. Screened yield is closely related and proportional to this chemical return. A high screened yield means the chemical return is high and a low screened yield means the chemical return is low. The second measurement of yield is fiber yield, by weight, per unit of wood. Rejects or screenings are related to and inversely proportional to the fiber yield. A high reject level means there is a low fiber return and a low reject level means there is a high fiber return. The total yield is the sum of these two yields. The ideal situation would be one in which there is a high chemical return and a high fiber return indicated by a high screened yield and low screenings.
The physical properties are freeness, burst, tear, fold, breaking length, density, and viscosity. Pulp samples are beaten in a PFI machine either for a specified number of revolutions and the freeness determined, or to a specified freeness and the time to freeness determined. The freeness of the pulp, Canadian Standard Freeness (CSF), is determined by TAPPI Standard T 227 M-58, revised August 1958. The beaten pulp is tested for burst, tear, fold, breaking length, and density. Burst is a numerical value obtained by dividing the burst strength in grams per square centimeter by the basis weight of the sheet in grams per square meter and is determined by TAPPI Standard Test T 220 M-60, the 1960 Revised Tentative Standard. This test is also used to determine tear. Tear is a numerical value and equals 100 e/r in which e is the force in grams to tear a single sheet, and r is the weight of the sheet per unit area in grams per square meter. Fold, breaking length in meters and density in grams per cubic centimeter are determined by TAPPI Standard Test T 220 OS-71. Pulp viscosity is in centipoises and is determined by TAPPI Standard Method T-230 SU-66.
There are two principal types of measurements to determine the completeness of the pulping or bleaching process, the degree of delignification and the brightness of the pulp. There appears to be no correlation between the two because the delignification factor is a measure of residual lignin within the pulp and the brightness is a measure of reflectivity of the pulp sheet.
There are many methods of measuring the degree of delignification of the pulp but most are variations of the permanganate test.
The normal permanganate test provides a permanganate or K number--the number of cubic centimeters of tenth normal potassium permanganate solution consumed by one gram of oven dry pulp under specified conditions. It is determined by TAPPI Standard Test T-214.
The Kappa number is similar to the permanganate number but is measured under carefully controlled conditions and corrected to be the equivalent of a 50% consumption of the permanganate solution in contact with the specimen. The test gives the degree of delignification of pulps through a wider range of delignification than does the permanganate number. It is determined by TAPPI Standard Test T-236.
PBC is also a permanganate test. The test is as follows:
1. Slurry about 5 hand-squeezed grams of pulp stock in a 600-milliliter beaker and remove all shives.
2. Form a hand sheet in a 12.5-centimeter Buckner funnel, washing with an additional 500 milliliters of water. Remove the filter paper from the pulp.
3. Dry the hand sheet for 5 minutes at 99.degree. to 104.degree. C.
4. Remove the hand sheet and weigh 0.426 grams of it. The operation should be done in a constant time of about 45 seconds to ensure the moisture will be constant, since the dry pulp absorbs more moisture.
5. Slurry the weighed pulp sample in a 1-liter beaker containing 700 milliliters of 25.degree. C. tap water.
6. Add 25 milliliters of 4 N sulphuric acid and then 25 milliliters of 0.1000 N potassium permanganate. Start the timer at the start of the permanganate addition.
7. Stop the reaction after exactly 5 minutes by adding 10 milliliters of the 5% potassium iodide solution.
8. Titrate with 0.1000 N sodium thiosulfate. Add a starch indicator near the end of the titration when the solution becomes straw color. The end point is when the blue color disappears.
In running the test, the thiosulfate should first be added as rapidly as possible to prevent the liberation of free iodine. During the final part of the titration the thiosulfate is added a drop at a time until the blue color just disappears. The titration should be completed as rapidly as possible to prevent reversion of the solution from occurring.
The PBC number represents the pounds of chlorine needed to completely bleach one hundred pounds of air dried pulp at 20.degree. C. in a single theoretical bleaching stage and is equal to the number of milliliters of potassium pemanganate consumed as determined by subtracting the number of milliliters of thiosulfate consumed from the number of milliliters of potassium permanganate added.
Many variables affect the test, but the most important are the sample weight, the reaction temperature and the reaction time.
There are also a number of methods of measuring pulp brightness. It usually is a measure of reflectivity and its value is expressed as a percent of some scale. A standard method is GE brightness which is expressed as a percentage of a maximum GE brightness as determined by TAPPI Standard Method TPD-103.
Another measure of the brightness or delignification is the opacity of the fiber. Opacity as a percent of a standard is determined by TAPPI Standard Test T 425 OS-75.
The cost of a pulping process is measured both by capital cost and operating chemical cost, the capital cost being the cost of the plant and facilities and the operating cost being the ongoing process costs of chemicals, raw materials and labor.
The capital costs of a mill are high because large vessels are required for holding the chips or pulp during each of the lengthy processing steps. For example, The Bleaching of Pulp states that the reaction times for a chlorine bleaching step is 45 to 90 minutes, for a hypochlorite bleaching step 1 to 8 hours, for a chlorine dioxide bleaching step 1 to 4 hours, and for an extraction step 1 to 2 hours. The size of a tower will depend upon the retention time, the rate of production, the effective tower volume, the consistency, the degree of packing, and the uniformity of the pulp flow. Rydholm, noted earlier, indicates that the space demand for one ton of pulp in a vessel will vary from 3 to 15 cubic meters, depending upon the consistency and degree of packing. He suggests a minimum of 7.5 cubic meters per ton of pulp and states that 7-8 cubic meters per tone of pulp is normal for medium-consistency operation. This probably is on an air-dry weight basis, the usual basis in the mill.
With this as background, we can now see how researchers attempted to add oxygen into the pulping and bleaching process, or to replace portions of the system with oxygen processing. A continuing concern was the poor solubility of oxygen in water and its poor transference from the gas to the liquid phase and into the fiber. The usual solutions to these problems have been high pressures, high oxygen concentrations, high pulp consistencies or particular vessel configurations to promote the transfer of oxygen into the fiber.
Several patents and articles describe bleaching sequences using oxygen.
Jamieson "The Present and Future Role of Oxygen Bleaching," undated, discloses a number of sequences using oxygen. These include C.sub.D OD, COD, OCED, OC.sub.D OD, ODEDED and OC.sub.D EDED.
Rerolle et al. U.S. Pat. No. 3,423,282 describe sequences having a central OC sequence. These are OCE, OCP and OCH.sub.(in situ). The C stage is reduced from 1 hour to 15 minutes and the amount of chlorine is reduced to 50-70% of the normal amount used.
Smith et al. U.S. Pat. No. 3,725,194 notes O.sub.2 CEDED, SO.sub.2 -O.sub.2 -SO.sub.2 -DED, So.sub.2 -O.sub.2 -H-DED, and SO.sub.2 -O.sub.2 -DED sequences.
Grigorescu "Oxygen Bleaching of Fibrous Pulps" Celuloza Si Hirtie 23 (2), 58-62 (1974) describes AODED, COD, CODED and OCDED sequences.
Jamieson et al. "Mill Scale Applications of Oxygen Bleaching in Scandinavia" 1973 TAPPI Alkaline Bleaching Pulping Conference, 231-238, lists a number of sequences. These are O, OP, OH, OD, ODED, COD, OCED, OC/DED, CODED, OC/DEHD, OCEDED, OC/DEDED, OHD, OPHD, OHPD, OC/DPD, OC/DE.sub.H D, and OC.
Jamieson et al. "Advances in Oxygen Bleaching" TAPPI, 11/71, 54, No. 11, 1903-1908 compares OC and CO sequences.
Soteland "Bleaching of Chemical Pulp with Oxygen and Ozone," Pulp and Paper Magazine of Canada, Vol. 75, No. 4, April 1974, pp. 91-96 mentions a number of sequences which include oxygen and high-consistency ozone treatments. These are oxygen-ozone, oxygen-ozone-peroxide, oxygen-ozone-hypochlorite, oxygen-ozone-ozone-peroxide, and oxygen-ozone-ozone-hypochlorite.
Rothenburg, et al. "Bleaching of Oxygen Pulps with Ozone," TAPPI, Vol. 58, No. 8, August 1975, pp. 182-185 describes oxygen-ozone, oxygen-ozone-sodium hydroxide extraction -ozone, oxygen-ozone-peroxide and oxygen-ozone-acetic acid sequences. The ozone treatment is high consistency in each of these sequences.
Kirk et al. "Low Consistency Oxygen Delignification in a Pipeline Reactor--Pilot Study," 1977 TAPPI Alkaline Pulping/Secondary Fibers Conference, Washington, D.C., November 7-10, 1977, describes a pipeline reactor in which pulp at a 3% consistency is bleached with oxygen using a waterfull system. Oxygen is introduced incrementally. In Table 2, Kappa numbers were measured at 15 minutes and 30 minutes. FIG. 4 is a graph of a Kappa number reduction versus time. Kappa numbers were measured at 3, 5, 10, 15, 20 and 30 minutes after oxygen addition.
The Rauma-Repola system is described in the Federal Republic of Germany Pat. No. 24 41 579, Mar. 13, 1975 in the Yrjala et al, New Aspects in Oxygen Bleaching, dated Apr. 18, 1974. The system uses the Vortex mixer shown in FIGS. 2 and 3 of the patent. It is possible, by using either a number of passes through a single mixer or several mixers in series, to bleach the pulp in from 5 to 15 minutes. The consistency is 3%.
Yrjala et al. "A new reactor for pulp bleaching" Kemian Teollisuus 29, No. 12: 861-869 (1972) describes a chlorine reactor.
Richter U.S. Pat. No. 4,093,506, issued June 6, 1978, describes a mixer for mixing bleaching fluids such as chlorine or chlorine dioxide with a high-consistency pulp. Rapidly rotating rotor blades essentially fluidize the pulp and the treatment gas is then added to it. The Kamyr reator is also described in an article, "Pilot and Commercial Results of Medium Consistency Chlorination," given at the Bleaching Seminar on Chlorination and Caustic Extraction, Nov. 10, 1977 in Washington, D.C.
The TAPPI monograph "The Bleaching of Pulp" describes and shows on pp. 325 and 332, respectively, single-shaft and double-shaft steam mixers. A steam mixer has a swept area of around 6500 square meters per metric ton of ovendry pulp.
Reinhall U.S. Pat. No. 4,082,233, issued Apr. 4, 1978, discloses a refiner having means for removing excess gas before the stock enters the refiner.